Planar silicon-on-sapphire composite

ABSTRACT

A semi-planar silicon-on-sapphire composite comprises a sapphire substrate, an epitaxial monocrystalline silicon mesa formed adjacent the substrate and an epitaxial deposition of monocrystalline aluminum oxide surrounding the mesa.

The Government has rights in this invention pursuant to Contract No.F19628-73-C-0146 awarded by the Department of the Air Force.

This application is a division of application Ser. No. 755,966, filedDec. 30, 1976, now U.S. Pat. No. 4,076,573.

The present invention relates to the manufacture of semiconductorsilicon-on-sapphire composites.

Specifically, the invention is useful in the manufacture ofsilicon-on-sapphire (SOS) devices wherein it is desirable to improveleakage current and other electrical characteristics of the devices byeliminating or diminishing diffusion into the sidewalls of the siliconmesa.

Silicon-on-sapphire substrates are usually non-planar. A typical surfaceconfiguration consists of an array of discrete single crystal siliconislands on a single crystal sapphire or alpha aluminum oxide substrate.Each silicon island or mesa presents a step of approximately 0.5 to 1.0micrometers to an interconnecting line traversing the sidewalls of thesilicon mesa. The line may be comprised of heavily doped silicon ormetal. Difficulty usually arises in covering or traversing the sidewallof the silicon mesa without experiencing reduced yields and subsequentfailures on extended life tests.

Sometimes several failure modes are manifested in the production ofsilicon-on-sapphire devices when the interface between the semiconductormesa and the substrate and the top edge of the semiconductor mesa arepermitted to be exposed to diffusion. One problem that can occur duringdiffusion is the erosion of the unprotected interface between the edgeof the semiconductor island and the substrate. The result is thecreation of discontinuities in conductive films later deposited incrossing relation to the island edge. On occasion, these discontinuitiescause a type of failure whereby the discontinuities result in opens inthe circuit path incorporating the particular line.

These discontinuities are also caused by the growth of gate oxide for adevice at temperatures below 1000° C. on a semiconductor island ofsilicon heavily doped with phosphorus. Under these conditions oxidegrowth occurs at an accelerated rate. The accelerated growth in theoxide thickness, coupled with the erosion of the sapphire substrate,causes undercutting of the edge of the silicon island. When the oxide atthe interface between the edge of the silicon and the sapphire substrateis removed, a discontinuity or gap is produced. One way of having aproper deposition of a film for the line onto the semiconductor islandor mesa on a substrate is to eliminate or diminish the sloping sidewallas a surface onto which deposition is made.

In particular, for silicon-on-sapphire devices a phosphorus dopant canreact with the sapphire substrate. This reaction has the result oferoding the sapphire at the abutting edge of the silicon island. Onemethod of preventing this kind of erosion and deformation of theinterface between the edge of the silicon island and the substrate is toprovide a protective barrier on top of the substrate and at theinterface by imbedding the silicon mesa in the substrate.

Another problem that can occur when diffusion is permitted through awindow which encompasses a sidewall of the semiconductor island is thecreation of an additional transistor device on the side of thesemiconductor island. This additional device is electrically connectedto the device formed on the top surface of the semiconductor island andis separated from the top surface by the top edge of the island. Byremoving each side from exposure to dopants during source and draindiffusion, this problem can be diminished or resolved and electricaloperation characteristics of a silicon-on-sapphire device formed fromthe composite significantly improved.

More so, silicon-on-sapphire technology has been plagued primarily bythree problems: the problem of the silicon island edge and itsassociated problems of instability; the problem of step coverage, i.e.depositing a dielectric or conductive layer onto the sidewall of thesilicon mesa where it extends from the substrate to a contact on a topsurface of the island, and the problem of dielectric strength weakeningfor the mesa sidewall coating. These problems associated with thesilicon island edge or sidewall and the character of the sidewalldielectric can be solved by eliminating the sidewall or edge, butprevious solutions used polycrystalline insulating material whichprevented the use of low-resistivity single crystal siliconinterconnections formed by epitaxy.

One method devised for eliminating diffusion into the sidewalls of thesilicon mesa formed in a sapphire substrate uses selective heteroepitaxywhereby monocrystalline silicon is grown on selected portions of thesapphire substrate. Elsewhere polycrystalline silicon is grown and thepolycrystalline silicon is used to isolate the various monocrystallinemesas formed on the substrate.

Another method for eliminating diffusion into the mesa sidewall is toselectively etch holes into a sapphire substrate, epitaxially growmonocrystalline silicon on the sapphire substrate and polish away theepitaxially grown silicon not within the holes or apertures in thesubstrate. One problem with this method is the extreme difficulty inproviding the holes in the sapphire substrate.

In the Drawings:

FIG. 1 depicts a cross-section of a composite of the invention;

FIG. 2 is a cross-section of a composite of a silicon mesa on a sapphiresubstrate;

FIG. 3 is an illustration of a cross-section depicting an intermediatestep in a method for making the composite of the invention; and

FIG. 4 shows apparatus useful for making the composite of thisinvention.

Shown in FIG. 1 is a composite 10 comprising a monocrystalline siliconsemiconductor mesa 12 epitaxially grown and attached to a substrate 14consisting of sapphire or alpha aluminum oxide, for example, inmonocrystalline form. There is also shown a layer 16 of monocrystallinealpha aluminum oxide, for example, epitaxially grown adjacent a major[1102] surface 19 of the sapphire substrate 14. The semiconductor mesa12 has at least one sidewall 20 surrounding a top surface 21 of the mesa12. A top edge 22 of the semiconductor mesa 12 is a juncture between thesidewall 20 and the surface 21. There is also another juncture which werefer to herein as the bottom edge 23 of the mesa 12. This bottom edge23 is a juncture between the mesa 12 and the substrate 14. Most of thesidewall 20 of the semiconductor mesa 12 is covered by monocrystallinealuminum oxide comprising the layer 16.

The composite 10 is useful for manufacturing integrated circuitscomprised of field effect transistors formed from or in the mesa 12 bydiffusion of impurities for the formation of sources and drains (notshown) therein. This aspect of the utility of the invention although nota part of the invention as disclosed herein is more fully described inU.S. Pat. No. 3,766,637, issued Oct. 23, 1973 to Norris et al. andassigned to RCA Corporation, entitled "Method of Making MOS Transistors"and said patent is hereby incorporated herein by reference thereto.

The composite 10 is preferably made by the method explained by referenceto FIGS. 2 and 3 of the drawings. As shown in FIG. 2, there is acomposite 24 comprised of the semiconductor mesa 12 of silicon adjacentthe aforementioned substrate 14 of sapphire, i.e., alpha aluminum oxide.There are many ways of forming single crystal silicon on a substratesuch as alpha aluminum oxide or sapphire. One method is describedbriefly in U.S. Pat. No. 3,393,088 issued July 16, 1968 to Manasevit etal. and entitled "Epitaxial Deposition of Silicon on Alpha Aluminum" andsaid patent is hereby incorporated herein by reference thereto. Asilicon semiconductor mesa such as the mesa 12 may be subsequentlyformed by photolithography as known in the art.

A unique aspect of this method is the growth of the epitaxial layer 16adjacent surfaces 19 and 21 of the composite 24 in such a fashion thatthe epitaxial layer 18 formed adjacent the monocrystalline silicon mesa12 and in particular adjacent the surface 21 is polycrystalline aluminumoxide (See FIG. 3). The particular epitaxial deposition consists ofalpha aluminum oxide, for example, which is deposited adjacent thesurfaces 19 and 21. The portion 17 is deposited simultaneously with theportion 18 in a single deposition step. The portion 17 is depositedadjacent the surface 19 of the sapphire substrate 14 as monocrystallinealuminum oxide or sapphire.

The growth rate of the portion 17 is approximately 31/2 times the growthrate of the portion 18 whereby a lesser thickness for portion 18 isachieved in comparison with the thickness of the portion 17 in the samegrowth period under similar growth conditions within the same growthperiod.

The growth of the layer 16 is of such a nature that because of theaccelerated growth rate of the monocrystalline aluminum oxide portion 17and the lesser growth rate of the portion 18 growing adjacent the topsurface 21, the portions 17 are founed to grow or extend part-way alonga portion of the sidewall 20 toward the top surface 21 of the siliconmesa 12.

The portion 18 of the layer 16 is removed from the adjacent surface 21of the silicon mesa 12 with hot phosphoric acid at a temperature ofapproximately 180° C. Typically, when the portion 18 is 0.35 micrometersthick an etching time in the phosphoric acid of approximately threeminutes is required. After removal of the portion 18, the composite 10,as shown in FIG. 1, is formed. A depression 25 is formed immediatelybelow the top edge 22 of the mesa 12. The depression 25 does not extendall the way down the side-wall 20 of the mesa so as to expose the bottomedge 23 of said mesa 12.

FIG. 4 shows a schematic drawing of an apparatus for carrying out thesimultaneous deposition of polycrystalline and monocrystalline alphaaluminum oxide as shown in FIG. 3 for the layer 16.

The apparatus comprises three gas tanks 26, 28, and 30 containing argon,hydrogen and carbon dioxide, respectively. The argon tank 26 has a line31 connected to a valve 32 and leading through a flowmeter 33 andanother valve 32 to a mixing chamber 36. The carbon dioxide tank 30 alsohas a line 38 connecting other valves 32 leading through anotherflowmeter 33 and valve 32 directly to the chamber 36. The hydrogen tank28 is connected via a valve 32 to a line 40 leading through a cold trap42 and then into a branch line 44 comprising a flowmeter 33 and valve 32and leading to the mixing chamber 36 and another branch line 46comprising another flowmeter 33 and valve 32 and leading to a means 48for sublimating aluminum chloride. A line 52 leads from mixing chamber36 through a valve 54 to a quartz reaction tube 56. A line 58,containing a valve 50, connects the mixing chamber 36 and a sublimator48. At the exit side of the sublimator 48 is a pipeline 60 surrounded bya heater 62. The pipeline 60 also leads to the reaction tube 56.

Inside the reaction tube 56 is a pedestal or susceptor 64. Outside thereaction tube 56 is a conventional radio frequency heater coil 66connected to a radio frequency generator (not shown). Leading from thereaction tube 56 is a vent 68.

In carrying out the present method of depositing aluminum oxide, thecomposite 24 which is at the manufacturing stage shown in FIG. 2 isplaced on the pedestal of the susceptor 64 and inserted in the reactiontube 56.

The system is then flushed with argon from tank 26 for several minutes.The argon passes through line 34 to the mixing chamber 36 from which itproceeds through line 58 to the sublimator 48 and thence through theline 60 to the reaction tube 56. It also proceeds from the mixingchamber 36 through the line 52.

The composite 24 is heated by the radio frequency heater coil 66. Theheating temperature may be varied within a range of about 875° C. and990° C., inclusive. With argon gas still flowing, hydrogen is permittedto flow from tank 28. The substrate temperature being between 875° and990° C. is critical to the simultaneous formation of polycrystallinealpha aluminum oxide adjacent single crystal silicon and the formationof monocrystalline aluminum oxide on the sapphire substrate 16. Thehydrogen flow is adjusted to about 2700 cc./min. and the temperature ispermitted to stabilize for five to ten minutes. When the hydrogen flowis established, the argon flow is shut off.

Meanwhile, aluminum trichloride in solid form is placed within a flask70 within the sublimator 48 prior to deposition and the flask 70 isheated at a temperature high enough to obtain a sufficient partialpressure of the aluminum trichloride. Aluminum trichloride has a vaporpressure of approximately 10 millimeters at a temperature of about 125°C. Hydrogen flowing through the line 46 to the sublimator 48 picks upthe aluminum trichloride vapor and carries it through the heated tube 60into the reaction tube 56. The tube 60 is heated so that aluminumtrichloride does not condense on its walls before reaching the reactiontube. The hydrogen, laden with aluminum trichloride vapor is adjusted toa flow rate of about 350 cc./min. for the line 46. The carbon dioxide(CO₂) flow is established through the line 38 to the mixing chamber 36.Hydrogen also flows through the line 44 to the chamber 36 at a similarrate where it is mixed with the carbon dioxide (CO₂).

The carbon dioxide flows through the line 38 to the mixing chamber 36and is adjusted to a flow rate of about 20 cc./min. The chemicalreaction which occurs is as follows:

    2 AlCl.sub.3 + 3H.sub.2 + 3CO.sub.2 → Al.sub.2 O.sub.3 + 6HCl = 3CO

the hydrogen and carbon dioxide react to form carbon monoxide and water,which is an intermediate product. The water immediately reacts with thealuminum trichloride to form alpha aluminum oxide and hydrogen chloride.

It has been found that there is an incubation period of about 30 secondsbefore an alpha aluminum oxide film begins to deposit on the composite24 as shown in FIG. 3. After about five to six minutes, an interferencecolor produced by the growing film becomes visible. A "straw" colorindicates a thickness of 400 Angstroms. A growth rate of 50 to 100Angstroms per minute is preferably maintained, but this can be widelyvaried between about 50 and 400 Angstroms per minute. However, it hasbeen shown that the lower growth rate of about 50 Angstroms per minuteproduces monocrystalline alpha aluminum oxide which is most uniform anddefect-free.

When the desired thickness of the layer 17 has been obtained, typicallya thickness of between 0.5 and 1.0 micrometers like that of the siliconmesa 12, the deposition is terminated by simultaneously shutting off thealuminum trichloride-hydrogen carrier flow and the carbon dioxide flow.

The radio frequency power is then turned off and the wafer is allowed tocool to room temperature, 28° C., for example. Argon gas is thensubstituted for hydrogen and the wafer is removed from the reaction tube56 for further processing. The gas line 52 may be used to assistflushing operations.

Further processing includes removal of the portion 18 of the depositedlayer 16 by immersion or etching in phosphoric acid at a temperature of180° C. for about three minutes, for example.

Using the method described above and the composite shown in FIG. 1,improved silicon-on-sapphire mesa transistors with lower leakage currentand highly stabilized current voltage characteristics may be produced.In addition, problems associated with erosion at the bottom edge 23between the silicon island 12 and the substrate 14 are eliminated bycovering this interface with a layer of monocrystalline aluminum oxide.

What is claimed is:
 1. A composite comprising an epitaxially grownsilicon island having at least one sidewall transverse to a majorsurface of said island, a monocrystalline aluminum oxide substratesupporting said island, and a layer of monocrystalline aluminum oxideadjacent to said substrate, said layer having a surface substantiallycoplanar with said major surface of said island, said layer surroundingsaid island in contiguous contact with said sidewall, whereby aninterface between the island and the substrate is covered.
 2. Acomposite as defined in claim 1 wherein said layer of monocrystallinealuminum oxide is epitaxially related to said substrate.
 3. An improvedsemiconductor structure of a silicon mesa having a top surface and asteep side wall transverse thereto deposited on a major surface of asapphire substrate, the improvement comprising:a layer ofmonocrystalline aluminum oxide deposited on the major surface of thesubstrate; the layer having a surface substantially co-planar with thetop surface of the mesa; the layer surrounding the mesa and incontiguous contact with the side wall thereof; and the layer having beenpyrohydrolytically formed by the reaction of aluminum trichloride,carbon dioxide and hydrogen at a temperature ranging between 857° C. and990° C.
 4. The improved structure of claim 3 wherein:the layer ofmonocrystalline aluminum oxide is epitaxially related to the substrate.